Gate 0: Jieitai—Ginzai nite, Kaku Tatakaeri

In the summer of 20XX, a "gate to another world'' suddenly appeared in Ginza, Tokyo. What came pouring in from inside was a mysterious army and a group of hideous monsters. They go around slaughtering people, and Ginza in broad daylight quickly turns into a hellscape. While everyone was helplessly thrown into confusion in this state of emergency, there was one man who was brave enough to move around. That man was Youji Itami, an otaku Self-Defense Force officer who would later be called the "Hero of Nijubashi"!

(Source: AlphaPolis, translated)